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(heavy-metal shadowing) A technique used in electron microscopy to determine the scale of surface features on a specimen. The specimen is partially coated with a thin layer of a heavy metal, e.g. platinum, palladium, or gold, which is evaporated in a vacuum chamber. The source of the metal ions is situated at a known angle to the specimen. Only the 'windward' side will be coated since metal atoms travel in straight lines. On placing the specimen in a beam of electrons, the uncoated 'leeward' side will allow freer passage of electrons and hence appear darker (a 'shadow') on the photographic plate. By measuring the length of the shadow cast it is possible, knowing the angle of the metal source, to calculate the height of the object casting the shadow. The technique has been used to gain more accurate measurements of very small specimens such as viruses.
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